1. Field of the Invention
The present invention relates to a method of manufacturing a phosphor screen of a cathode ray tube.
2. Description of the Related Art
As a color cathode ray tube of high contrast, there is known a color cathode ray tube in which a panel having a phosphor screen formed on its inner surface is made of a dark glass having a transmittance of about 40% to 45% or a color cathode ray tube in which a phosphor screen is formed of a so-called pigment phosphor in which a pigment of the same color is deposited on a phosphor particle.
In the former color cathode ray tube in which the panel is formed of the dark glass, an external light is absorbed by the dark glass so that a contrast thereof can be increased. At the same time, however, a light emitted from the phosphor screen also is absorbed by the dark glass so that a brightness thereof is lowered.
In the latter cathode ray tube in which the phosphor screen is formed by using the pigment phosphor, an external light is absorbed by the pigment so that a contrast thereof can be increased. In this case, however, phosphor particles of several layers (e.g., three to four layers) are laminated to form the phosphor screen. As a result, a light from the phosphor particle is partly absorbed by the pigment with the result that a brightness thereof is lowered about 10 to 15%.
In order to improve the aforesaid defects, there has been developed a color cathode ray tube in which a color filter layer is formed on the inner surface of the panel and a phosphor layer is formed on the color filter layer to form a phosphor screen.
In this color cathode ray tube, a glass having a high transmittance can be utilized as a panel glass and an amount of phosphor can be increased as compared with the pigment phosphor. Also, external light is absorbed by the color filter layer so that a high contrast and a high brightness can be obtained.
The color filter layer for the phosphor screen is formed as follows:
A photosensitive liquid made of polyvinyl alcohol (PVA) and ammonium dichromate (ADC) is coated with a slurry in which a fine particle inorganic pigment is dispersed, dried, exposed with ultraviolet rays by using a color selecting electrode as a photo mask and then developed to thereby form the color filter layer.
In the above method of forming the color filter layer, upon exposure, ultraviolet rays are absorbed by the pigment and the transmittance of ultraviolet rays is decreased. As a result, a boundary that is in contact with the glass panel is not exposed sufficiently so that a loose failure of the color filter layer occurs. If an exposure amount is increased, then a width of color filter layer is increased and a predetermined width cannot be obtained.